Process for removing carbides and other impurities from slags



Patented May 3, 1938 PATENT orrlcr.

PROCESS FOR REMOVING OARBIDES AND OTHER IMPURITIES FROM SLAGS ErnstKarwat, Grosshesselohe, near Munich, Germany, assignor, by mesneassignments, to Union Carbide and Carbon Corporation, a corporation ofNew York No Drawing. Application October 24, 1936, Se-

rial No. 107,369. In Germany October 21,

6 Claims. (01. 106-255) The invention relates to the manufacture ofslag-cement having the approximate composition CaO-60% to 65% MgOup to3% Fe2O3--2% t A12O35% to SiO2-2O% to 25% Blast furnace slags producedin the ordinary processes of manufacture of pig iron have theapproximate chemical composition:

CaO40% to 45% MgO5% to 10% Al2Os-10% to 20% SiO235% to 40% Customarily,slag-cement of the Portland type is produced from such blast furnaceslags either by grinding the solid slag with proper added proportions ofslaked lime and possibly other in- 30 gredients; or by calcining theground slag with limestone and possibly other ingredients; or by addingto the molten slag, and melting into it, lime and possibly otheringredients. Each of these methods has for its object to bring about 35a substantial change in the chemical composition and hydraulicproperties of the original blast furnace slag, so that the ultimateproduct shall have the approximate composition and hydraulic propertiesof Portland cement.

40 Recently it has been found that under certain conditions slag ofapproximately Portland cement composition can be produced directly inblast furnaces simultaneously with the smelting of pig iron or the like.Generally speaking, this is ac- 5 complish-ed by greatly increasing theproportion of limestone in the charge, and increasing the hearthtemperature of the furnace. As the melting point of such a Portlandcement slag is about 1900" C., as compared with a melting point 50 ofabout 1400" C. to 1500" C. for ordinary blast furnace slag, it isevident that if a slag of true Portland cement composition is to beproduced,

the hearth temperature of the furnace must be increased to a temperaturefar above normal 55 last furnace temperatures. Such a high temperaturecan be attained by expedients such as the use of oxygen-enriched blast.It has been found that slag-cement produced under such high temperatureconditions usually contains a considerable proportionsometimes two orthree 5 percentum-of calcium carbide, an impurity not ordinarily foundin cement of any type produced by methods other than the one justdescribed.

Calcium carbide, when present in cement in such large proportion, isextremely objectionable 10 because when the cement is slaked the calciumcarbide reacts with water to evolve acetylene, which is dangerousbecause of its inflammable properties, and may ruin the cement castingby producing porosity. If slag-cement of the Port- 15 land cement typeis to be produced successfully by the direct smelting method, means foreliminating or destroying the calcium carbide contained in it must beprovided.

The present invention is based upon my dis covery that certain salts ofthe metals of the alkali and alkaline earth groups, when added to themolten slag-cement in relatively small proportions, effectively destroyand render harmless the calcium carbide contained in the slag- 25cement, without otherwise substantially altering either its compositionor its hydraulic properties.

Substances which I have found eiiective for this purpose are thehalides, carbonates and sulphates of the alkali metals and alkalineearth metals.

For example, a sample of slag-cement which contained about 2% of calciumcarbide was treated in the molten state with 4% by weight of sodiumchloride, and a similar sample was treated with about 4% by weight ofmagnesium chloride. In each instance, calcium carbide could not bedetected in the treated slag-cement, though untreated material evolvedgas in copious quantities when moistened with water. Similar samples ofslag-cement, treated in the molten state with calcium fluoride, sodiumcarbonate, and calcium sulphate, likewise showed substantially completeelimination of calcium carbide by the treatment.

The reaction of the salts with the calcium carbide of the slag isextremely rapid and complete, and it is suflicient to add the salts tothe liquid slag on tapping, shortly after its separation from the metal.Most of the salts and their products of reaction are volatile andtherefore have no influence on the hydraulic properties of the slag.

If the slag is well granulated with air, it is not necessary to'destroyall of the calcium carbide by adding salts. In such a case it issufficlent to decompose only apart of the carbide with salts and tooxidize the remainder in the granulating step.

While the invention has been described in connection with the removal ofcalcium carbide from directly-smelted slag-cements i of the Portlandtype, I have found that calcium phosphide is also sometimes present insuch slag-cements in objectionably large proportions, and that themethod of the invention is likewise effective in destroying or renderingharmless calcium phosphide contained in such slag-cement.

This application is in part a continuation of my United States patentapplication Serial No.

693,398, filed October 12, 1933;

I claim:

1. Process for treating iron blast furnace molten slag containing anamount of carbide in ex' cess of that which can be destroyed by airgranulation of the slag, which process comprises adding to the moltenslag after its separation from the metal a carbide-destroying saltchosen from the group consisting of the halides, carbonates, andsulfates of the alkali and alkaline earth metals, the amount of saidadded salt being enough to destroy said excess carbide but notsubstantially more than enough to destroy all of the carbide in theslag.

2. Process as defined in claim 1 in which the salt is a chloride.

1 in which the 1 in which the ,salt added to the slag is less than issufficient alone to decompose the carbide content of the slag.

ERNST KARWAT.

